First of all, do not fret. "Nausicaa" is one of the easier episodes in Ulysses, and then right on its heels comes "Oxen of the Sun," quite possibly the hardest. Joyce has brought all of his stylistic powers of imitation to bear in the chapter, and the result is that the prose can be extremely dense and hard to parse. But style is the thing that makes "Oxen of the Sun" one of the most remarkable chapters in the novel.
What is Joyce up to here? "Oxen of the Sun" is written in over twenty different styles, each one parodied chronologically. The episode begins with literal translations of early Latinate prose, moves into Anglo-Saxon alliterative verse, then the Renaissance works of various authors, then up into 18th and 19th century novels. In the last few pages, the style dissolves into a mix of Irish dialects and Dublin slang. (If you check out the detailed summary of the chapter, each style is listed in order along with the action in the chapter that it corresponds to). In short, Joyce provides a stylistic guide to the history of Western literature (with a strong Irish emphasis, no doubt) in just over 40 pages.
Furthermore, Joyce attempts to demonstrate the ways in which style is utterly inseparable form content: how you write about something determines what you write about. If you read the episode over a couple of times, you'll begin to notice how a given writing style moves over the scene like a magnet and picks up the pieces that it is most prone to focus on.
For example, the early Latinate prose, always trying to get a taste of history, speaks in generalities of the powers of procreation and the proud Irish medical tradition, while the Anglo-Saxon alliterative verse is used almost as baby-talk to imagine the state of bliss before an infant is delivered: "Before born babe bliss had. Within womb won he worship" (14.7). The style of the morality play Everyman is used to capture the seriousness with which Nurse Callan describes the difficulty of Mina Purefoy's birth. As Bloom judges the other men for being too rowdy in the hospital, the hypocritical nature of his thoughts is rendered in the savage 18th-century satirical style of Junius. Finally, the drunken cross-talk of the men in Burke's pub is rendered in Dublin slang and colloquialisms. The didactic (preachy) point: form is content; content is form.
Joyce once wrote in a letter to a friend (probably half-seriously) that in this episode, "Bloom is the spermatozoon, hospital the womb, the nurses the ovum, Stephen the embryo." What he was getting at is that the stylistic effect in the chapter is a metaphor for the process of gestation that takes place during pregnancy. The episode is set in a maternity ward while the men wait to hear news of Mina Purefoy's birth. When Mulligan comes in, they actually joke about how fat he is and suspect that he too is pregnant with child. Yet the chapter suggests that the male creative process is different than that of the female. As the artist's prose style moves through the evolution of prose style in literary history, it seems to be suggesting that men give birth to books, not children. All Stephen's moodiness surrounding his ambition to write a great masterpiece might be something like a mother's labor pains. And in Ulysses, Stephen, like Mina Purefoy, is having a hard time giving birth.
"Oxen of the Sun" corresponds to Book 12 of the Odyssey, where Odysseus and his men land on the island of Helios, the sun-god. Odysseus has been warned not to harm the cattle on the island, and asks his crew to swear the same. When he falls asleep one of the crew members, Eurylochus, convinces the men to forget their oath and to slaughter the cattle. Odysseus wakes up in despair, but realizes that there is nothing to be done. After seven days on the island, they depart, but Helios appeals to Zeus, who promptly destroys their ship with a lightning bolt. Odysseus lashes the mast to keel of his shattered ship and endures the voyage alone.
There are a number of small correlations throughout the Ulysses episode. For one, Bloom has just woken up from his nap in "Nausicaa," like Odysseus on the island. For another, the men are mocking women and the process of giving birth despite Bloom's calls for moderation, just as Odysseus's crew slaughtered the cattle despite Odysseus's warnings. Zeus's thunderbolt here comes in the form of a thunderclap that makes Stephen fear that the gods have heard his blasphemy. At the end of the episode, the prose seems to be extremely "stormy" and hard to get a handle on, and Bloom is very much alone.
The key correlation, however, is between the oxen and fertility. All the men discuss pregnancy at length, but they often make a joke of it, and never think to empathize with the pain that the women are going through. Bloom, by contrast, "felt with wonder women's woe in the travail that they have of motherhood" (14.13). Bloom alone amongst the men recognizes the sanctity of womanhood just as Odysseus is the only one to recognize the sacredness of the cows.
On a simpler thematic level, Bloom and Stephen are drawn more closely together in "Oxen of the Sun" than at any previous point in the book. Bloom observes Stephen as they sit there in the room. He has a number of fatherly feelings for Stephen: he takes pity on him, thinks he is wasting his time drinking and spending time with prostitutes, and suspects that the calm demeanor Stephen puts on is blatantly false. When Stephen is scared by the thunderclap, it is Bloom who tries to calm him.
As Bloom begins to act fatherly toward Stephen, we also see how the two are twinned in their loneliness. Bloom is roundly ignored in the room, and when he attempts to launch into a scientific discussion of infant mortality, the men quickly begin talking over him. In "Aeolus," Stephen appeared in his element and was surrounded by admiring newspapermen. Here, by contrast, we see Stephen in a more typical state. He pontificates about religious matters at length and is kidded by the other young men for being overly ambitious and not achieving anything. As Stephen and Bloom become more outcast by the group, they drift closer and closer together.